What is the definition of not paying attention?

Paying attention means being watchful, concentrating on what is being said and what is going on in your surroundings or what you are doing.
Not means doing the opposite-- that is, letting your mind wander, thinking about something other than what is being said or what is going on where you are, daydreaming, maybe even sleeping or dozing off, failing to concentrate on what you are doing or should be doing.
Paying attention means being watchful, concentrating on what is being said and what is going on in your surroundings or what you are doing.
Not means doing the opposite-- that is, letting your mind wander, thinking about something other than what is being said or what is going on where you are, daydreaming, maybe even sleeping or dozing off, failing to concentrate on what you are doing or should be doing.